Designing Out Damp and Mould, For Good

Damp and mould in social housing is not new, nor is it unsolvable. Despite decades of habit to the contrary, it is mostly not a resident behaviour problem. It is a design problem, and design problems have design solutions.

The Housing Ombudsman's 2021 report was titled It's Not Lifestyle¹ for good reason. Attributing damp and mould to residents rather than to the homes they live in was found to be so prevalent it was called systemic. The death of two-year-old Awaab Ishak, whose landlord blamed damp and mould on "ritual bathing", put that failure in its starkest light.

The law has since shifted decisively. Awaab's Law came into force in England in October 2025; Wales' equivalent hazard rule under the Welsh Housing Quality Standard 2023 went live in April 2026². In England, damp and mould growth is a defined hazard under the Housing Health and Safety Rating System, which underpins the reformed Decent Homes Standard, an updated standard that applies in full from 2035². Just as important, and often overlooked: condensation damp usually falls outside a landlord's repairing covenant, because it is not "disrepair". But under the Homes (Fitness for Human Habitation) Act 2018, which added section 9A to the Landlord and Tenant Act 1985, a home must be fit to live in regardless of whether anything is in disrepair³. Damp and mould is actionable on that basis alone. Compensation has followed: in 2024–25, over 40 per cent of the redress ordered by the Ombudsman related to leaks, damp and mould, with the largest single award close to £32,000¹.

Meeting a statutory response time is necessary. It is not sufficient. Investigating within ten working days does not insulate a cold wall, balance a ventilation system, or match a home's moisture capacity to the family living in it. The only way to fix damp and mould for good is to address its root causes: in the brief, in the specification, in the tenancy, and in the handover. Here is how I believe we can design this problem out for good:

Step 1Know your residents

Every home produces moisture through daily living. Breathing, cooking, bathing, drying clothes. A family of four generates around 10 to 15 litres a day from normal activity, and closer to 20 if clothes are dried indoors. The World Health Organisation is explicit that moisture control "should be addressed in the early phases of building construction" through design that minimises mould growth⁴. That is a design responsibility, not a resident one.

But social housing is not occupied uniformly. Before writing a brief, social landlords should understand their own residents from this perspective:

• Moisture load. What share of your residents cook from scratch, dry clothes indoors, or use the home intensively all day? Larger families and those in fuel poverty do all three more often. And check the provision: if your design assumes clothes are not dried indoors, are you genuinely offering a year-round alternative you would use yourself?
• Heating behaviour and fuel poverty. Mould is not only a ventilation problem. Where a surface falls below the dew point of the indoor air, condensation forms whatever the ventilation rate. When residents cannot afford to heat, surfaces cool and condensation blooms. If underheating is likely in your stock, the ventilation strategy has to work harder, and the fabric specification must minimise cold-bridge risk.

Step 2Design for real occupancy, not a national average

This is the most technically important step, and the one I have seen fewest landlords take. The Standard Assessment Procedure (SAP 10.2), used to design ventilation in most housing, estimates occupancy from floor area using a formula unchanged since 2009 and calibrated against survey data from 2002–05⁵. For a typical five-bedroom house of around 130m², SAP assumes about 2.9 occupants. The government's own Future Homes Standard technical work acknowledges the problem, and SAP's documentation states plainly that its assumed occupancy "is not suitable for design purposes".

A social landlord does not have to design to a national average that ignores its own stock. It controls who moves in, through an allocation policy almost universally based on the Bedroom Standard⁶: one bedroom per couple, one per adult aged 21 or over, one per pair of same-sex adolescents, one per pair of children under ten. A household allocated five bedrooms under that standard is likely to be two parents and three to five children, so six to eight people. The ventilation, designed to SAP in the absence of a correction, would be sized for fewer than three. That gap between design assumption and allocation reality is where damp and mould begins.

The building regulations already provide the mechanism, and it is worth noting the two nations diverge:

• England. Approved Document F (2021) sets whole-dwelling ventilation rates by bedroom count (19, 25, 31, 37 and 43 l/s for one to five bedrooms), adding 6 l/s per additional bedroom, or 0.3 l/s per m², whichever is greater⁷.
• Wales. Approved Document F (2022) keeps an occupant-based uplift: a default of two people in the main bedroom and one in each other, plus 4 l/s per additional occupant where greater occupancy is expected⁷.

Either way, the Bedroom Standard gives you the number to design to. What has been missing is the expectation that landlords use the two together. And a note on retrofit: PAS 2035 requires an occupancy assessment, which is welcome, but it captures a snapshot of the household present on the day⁸. Tenancies change; a home retrofitted around five people today may house eight in two months.

Hence the Retrofit Assessment should inform commissioning for the current household (where installed kit might be "tuned down"), but use the Bedroom Standard to define the design envelope and safety tolerance, so that the building can handle any future allocation. Plus, if your data shows persistent overcrowding, and 8.9 per cent of social rented homes are overcrowded against 1.0 per cent of owner-occupied⁹, make sure that safety tolerance on the Bedroom Standard factors that in too.

Step 3Specify it in the brief

Good buildings begin with good briefs. Combining your resident profile (Step 1) with your occupancy assumption (Step 2) gives designers evidence-based requirements. Without them, designers default to SAP; with them, they cannot. Build in a sensible safety factor for the risk of overcrowding.

For new build, specify whole-dwelling ventilation rates based on Bedroom Standard maximum occupancy, fabric that minimises cold bridging and fuel bills, and heating that reaches adequate surface temperatures even under cautious use. A special mention is worthwhile for homes acquired through Section 106: set this brief before you buy. If the developer has not corrected for occupancy, you are almost certainly buying an under-ventilated home that will create problems for you to deal with later.

For retrofit, the trigger is lower than many landlords assume. Approved Document F applies whenever building work could affect ventilation, and its governing principle is that ventilation must be left no worse than before⁷. Replacing windows increases airtightness and usually strips out the old draught paths, so trickle vents of at least 8,000mm² equivalent area must be reinstated in habitable rooms; adding insulation or improving airtightness cuts background infiltration; and refurbishing a bathroom or kitchen brings extract requirements into play. These are routine maintenance works, not major projects, and each is a moment to assess ventilation against the brief, not the under-specified baseline you inherited. The wider moisture picture, from interstitial condensation to rain penetration, is covered by BS 5250, whose whole-building approach makes moisture a design discipline rather than an afterthought¹⁰.

On strategy, specify a preferred hierarchy rather than leaving system choice to cost alone. Each social landlord will bring their own experience to this, but as a starting point I'd suggest:

• Mechanical ventilation with heat recovery (MVHR) for high-occupancy, high-risk homes. It recovers 85 to 95 per cent of the heat in the extracted air, so designing to maximum occupancy carries little heat penalty, and it can be recalibrated at change of tenancy to the household moving in.
• Humidity-sensitive, demand-controlled continuous extract with boost (dMEV) as the sensible default. It increases airflow as humidity rises and eases off as it falls, self-calibrating without manual intervention. In a temperate climate and typical stock its running energy is comparable to MVHR, though in very airtight homes MVHR's heat recovery gives it the clear edge. Traditional dMEV with a recalibration step at void works too, provided a boost function is included.
• Natural and intermittent ventilation as a last resort only. A strategy that leans on opening windows and intermittent extract fans has been widely proven unable to manage moisture reliably in an occupied home.

Step 4Tell residents what you assumed

A well-specified building is necessary. It is not sufficient. The operating envelope of the home has to reach the people living in it, in language they can act on, at the moment they need it.

For every letting, provide a one-page, property-specific "How Your Home Works" sheet: how many people the home is designed for, the level of cooking, bathing and indoor drying it can handle, and the ventilation system provided to manage it. This is not telling residents how to live. It is stating the scope the home was designed to cope with, and inviting them to say if they live beyond it, so you can adjust. Cover what the system is, where the controls are, and who to call if it breaks. Approved Document F already requires equivalent documentation for new-build buyers within five days of completion; tenants deserve the same⁷. The GOV.UK Existing Home Ventilation Guide and the CIH/NIFHA damp and mould guide are strong, accessible models.

Put the operating expectations into the tenancy as plain-language terms: keep trickle vents open unless told otherwise, do not obstruct or disable ventilation, report any fan or control that stops working. Awaab's Law guidance is clear that a landlord cannot rely on a behaviour-based defence unless it can point to a contractual breach, and a breach needs a term. You cannot hold someone to a standard you never communicated.

Then build the loop into the void process, with three non-negotiable steps before any new tenancy:

• Recalibrate the ventilation to the incoming household's expected occupancy.
• Confirm trickle vents are clear, extract fans work, and no terminals are blocked or painted over.
• Hand over the property-specific guide in person, and document it in the tenancy file.

When a damp issue does arise and Awaab's Law requires a written response, use that letter to explain how the home works and what you will do together, not to list what the resident did wrong.

In summary

Damp and mould are not random defects. They are the predictable result of designing homes to an average that never reflected who lives in social housing, of ventilation systems that were never commissioned, maintained or explained, and of attributing the consequences to the residents with the least power to change them.

Fixing it for good asks social landlords to act as the informed, responsible clients they are capable of being. They know their residents. They know their allocation policies. They have the regulatory frameworks, the technology and the good-practice guidance. What has been missing is the line drawn between those dots, and the recognition that drawing it is both a professional obligation and a legal one. The buildings already tell us what happens when we get this wrong. It is time to design homes that work for the people we know will live in them.

References

1. Housing Ombudsman, Spotlight on damp and mould: it's not lifestyle (2021); Annual Complaints Review 2024–25.
2. Social Housing (Regulation) Act 2023 (Awaab's Law) and the reformed Decent Homes Standard, built on the Housing Health and Safety Rating System (HHSRS), England; Welsh Housing Quality Standard 2023 hazard rule, Wales.
3. Homes (Fitness for Human Habitation) Act 2018, inserting section 9A into the Landlord and Tenant Act 1985.
4. World Health Organization, Guidelines for Indoor Air Quality: Dampness and Mould (2009).
5. Standard Assessment Procedure (SAP) 10.2, BRE for the Department for Energy Security and Net Zero.
6. Bedroom Standard, in Allocation of accommodation: guidance for local housing authorities (MHCLG, 2012).
7. Approved Document F (Ventilation), Volume 1: England (2021) and Wales (2022).
8. PAS 2035, Retrofitting dwellings for improved energy efficiency.
9. The Health Foundation, Trends in household overcrowding by tenure (English Housing Survey 2023–24).
10. BS 5250:2021, Management of moisture in buildings. Code of practice.